As shown in FIG. 14, a magnetic memory cell 105 of a magnetic memory (magnetic random access memory: MRAM) with a conventional magnetoresistance effect element has a structure where a magnetoresistance effect element 110 and a selection transistor 109 are electrically connected in series. The selection transistor 109 has a source electrode electrically connected to a source line 102, a drain electrode electrically connected through the magnetoresistance effect element 110 to a bit line 104, and a gate electrode electrically connected to a word line 103. The magnetoresistance effect element 110 basically has a three-layer structure with a first ferromagnetic layer 111, a second ferromagnetic layer 112, and a non-magnetic layer 113 provided between the first and second ferromagnetic layers 111 and 112. The resistance value of the magnetoresistance effect element 110 is reduced if the respective magnetizations of the first and second ferromagnetic layers 111 and 112 are parallel and is increased if these magnetizations are antiparallel. The memory cell of the MRAM assigns these two resistance conditions to bit information “0” and bit information “1”.
In the MRAM, the magnetoresistance effect element 110 becomes finer and finer year by year for a higher integration. Both the first and second ferromagnetic layers 111 and 112 of the magnetoresistance effect element 110 are in the risk of thermal agitation of their magnetizations as these layers become finer, potentially leading to loss of bit information. To retain bit information even if the first and second ferromagnetic layers 111 and 112 become finer, the second ferromagnetic layer 112 to become a recording layer is required to have a thermal stability factor (E/kBT) of 70 or more and the first ferromagnetic layer 111 to become a reference layer is required to have a thermal stability factor (E/kBT) higher than that of the second ferromagnetic layer 112. Here, E is an energy barrier required for magnetization reversal and corresponding to the product of a magnetic anisotropy energy density Keff and a volume V of the first or second ferromagnetic layer 111 or 112 (E=KeffV), kB is a Boltzmann's constant, and T is the absolute temperature.
To obtain high thermal stability expressed as E/kBT, the effective magnetic anisotropy energy density Keff of the first or second ferromagnetic layers 111 or 112 should be increased. In terms of this aspect, a perpendicular magnetic anisotropy magnetoresistance effect element giving an easy axis of perpendicular magnetization to the first or second ferromagnetic layer 111 or 112 has received attention. Rare earth based amorphous alloys, L10-ordered element (Co or Fe)—Pt alloys, Co/(Pd or Pt) multilayer films, etc. have been studied as electrodes of such perpendicular magnetic anisotropy (see non-patent literature 1, 2, or 3, for example).
The present inventors have found that, in a stacked structure of CoFeB/MgO, reducing the thickness of the CoFeB layer generates perpendicular magnetic anisotropy (see patent literature 1, for example). By applying the stacked structure of CoFeB/MgO to a perpendicular magnetic anisotropy magnetoresistance effect element, E/kBT of substantially 40 is obtained in the second ferromagnetic layer 112 as a recording layer with a junction size diameter of 40 nm (see non-patent literature 4, for example). Further, by forming a double CoFeB—MgO interface recording layer structure and increasing the thickness of a magnetic layer in a recording layer with the purpose of enhancing thermal stability, E/kBT of 80 or more is obtained with the junction size diameter on the order of 40 nm and E/kBT of substantially 59 is obtained with the junction size diameter of 29 nm in the second ferromagnetic layer 112 as a recording layer (see non-patent literature 5, for example). The junction size of a ferromagnetic layer mentioned herein is the length of a longest straight line on a junction surface of the ferromagnetic layer at which the ferromagnetic layer contacts an adjacent non-magnetic layer or an adjacent electrode. According to non-patent literatures 4 and 5, the ferromagnetic layer has a circular columnar shape and the junction surface has a circular shape. Thus, the junction size means the diameter of the junction surface.
As shown in FIG. 15(a), a perpendicular magnetic anisotropy magnetoresistance effect element described in non-patent literature 4 has a basic structure where a lower non-magnetic electrode 114 and an upper non-magnetic electrode 115 are connected to a three-layer structure including a first ferromagnetic layer 111, a second ferromagnetic layer 112, and a non-magnetic layer 113 provided between the first and second ferromagnetic layers 111 and 112. As shown in FIG. 15(b), the second ferromagnetic layer 112 as a recording layer characteristically has a junction size D larger than a ferromagnetic layer thickness t.
As shown in FIG. 15(c), a perpendicular magnetic anisotropy magnetoresistance effect element described in non-patent literature 5 has a five-layer structure where a second non-magnetic layer 116 is stacked on a second ferromagnetic layer 112 on a non-magnetic layer 113, a third ferromagnetic layer 117 is stacked on the second non-magnetic layer 116, and a third non-magnetic layer 118 is stacked on the third ferromagnetic layer 117. Interface magnetic anisotropy is generated between the third ferromagnetic layer 117 and the third non-magnetic layer 118. Non-patent literature 5 recites that use of this five-layer structure can enhance thermal stability. This structure still has a characteristic in that a total thickness t of a recording layer including the second and third ferromagnetic layers 112 and 117 magnetically coupled through the second non-magnetic layer 116 is smaller than a junction size D.